Trailing Edge Noise Reduction by Passive and Active Flow Controls

This paper presents the results on the use of porous metal foams (passive control) and dielectric barrier surface plasma actuations (active control) for the reduction of vortex shedding tonal noises from the nonflat plate type trailing edge serration in a NACA0012 airfoil previously discussed in Chong et al. (AIAA J. Vol. 51, 2013, pp. 2665-2677). The use of porous metal foams to fill the interstices between adjacent members of the sawtooth can almost completely suppress the vortex shedding tonal noise, whilst the serration effect on the broadband noise reduction is retained. This concept will promote the nonflat plate type serrated trailing edge to become a genuine alternative to the conventional flat plate type serrated trailing edge, which is known to have drawbacks in the structural stability, aerodynamic performances and implementation issues. For the plasma actuators, configuration which produces electric wind in a tangential direction is found to be not very effective in suppressing the vortices emanated from the serration blunt root. On the other hand, for the plasma configuration which produces electric wind in a vertical direction, good level of vortex shedding tonal noise reduction has been demonstrated. However, the self noise produced by the plasma actuators negates the noise benefits on the tonal noise reduction. This characteristic illustrates the need to further develop the plasma actuators in a two pronged approach. First is to increase the electric wind speed, thereby allowing the plasma actuators to be used in a higher free jet velocity which naturally produces a larger level of jet noise. Second, the self noise radiated by the plasma actuators should be reduced

Controlled sign reversal of electroresistance in oxide tunnel junctions by electrochemical-ferroelectric coupling

The persistence of ferroelectricity in ultrathin layers relies critically on screening or compensation of polarization charges which otherwise destabilize the ferroelectric state. At surfaces, charged defects play a crucial role in the screening mechanism triggering novel mixed electrochemical-ferroelectric states. At interfaces, however, the coupling between ferroelectric and electrochemical states has remained unexplored. Here, we make use of the dynamic formation of the oxygen vacancy profile in the nanometerthick barrier of a ferroelectric tunnel junction to demonstrate the interplay between electrochemical and ferroelectric degrees of freedom at an oxide interface. We fabricate ferroelectric tunnel junctions with a La_0.7Sr_0.3MnO_3 bottom electrode and BaTiO_3 ferroelectric barrier. We use poling strategies to promote the generation and transport of oxygen vacancies at the metallic top electrode. Generated oxygen vacancies control the stability of the ferroelectric polarization and modify its coercive fields. The ferroelectric polarization, in turn, controls the ionization of oxygen vacancies well above the limits of thermodynamic equilibrium, triggering the build up of a Schottky barrier at the interface which can be turned on and off with ferroelectric switching. This interplay between electronic and electrochemical degrees of freedom yields very large values of the electroresistance (more than 10^6% at low temperatures) and enables a controlled switching between clockwise and counterclockwise switching modes in the same junction (and consequently, a change of the sign of the electroresistance). The strong coupling found between electrochemical and electronic degrees of freedom sheds light on the growing debate between resistive and ferroelectric switching in ferroelectric tunnel junctions, and moreover, can be the source of novel concepts in memory devices and neuromorphie computing

Ferroionic inversion of spin polarization in a spin-memristor

Magnetoelectric coupling in artificial multiferroic interfaces can be drastically affected by the switching of oxygen vacancies and by the inversion of the ferroelectric polarization. Disentangling both effects is of major importance toward exploiting these effects in practical spintronic or spinorbitronic devices. We report on the independent control of ferroelectric and oxygen vacancy switching in multiferroic tunnel junctions with a La_(0.7)Sr_(0.3)MnO_3 bottom electrode, a BaTiO_3 ferroelectric barrier, and a Ni top electrode. We show that the concurrence of interface oxidation and ferroelectric switching allows for the controlled inversion of the interface spin polarization. Moreover, we show the possibility of a spin-memristor where the controlled oxidation of the interface allows for a continuum of memresistance states in the tunneling magnetoresistance. These results signal interesting new avenues toward neuromorphic devices where, as in practical neurons, the electronic response is controlled by electrochemical degrees of freedom

Tight-binding approach to penta-graphene

We introduce an effective tight-binding model to discuss penta-graphene and present an analytical solution. This model only involves the π-orbitals of the sp2-hybridized carbon atoms and reproduces the two highest valence bands. By introducing energy-dependent hopping elements, originating from the elimination of the sp3-hybridized carbon atoms, also the two lowest conduction bands can be well approximated - but only after the inclusion of a Hubbard onsite interaction as well as of assisted hopping terms. The eigenfunctions can be approximated analytically for the effective model without energy-dependent hopping elements and the optical absorption is discussed. We find large isotropic absorption ranging from 7.5% up to 24% for transitions at the Γ-point

Structural, electronic and magnetic properties of the surfaces of tetragonal and cubic HfO_(2)

We present ab initio density-functional theory (DFT) calculations of the structure and stability of the monoclinic (m), tetragonal (t) and cubic (c) phases of HfO_(2) and of the stability and the structural, electronic, and magnetic properties of the polar (001) surface of t-HfO_(2) and the (100) and (111) surfaces of c-HfO_(2). We show that on all three surfaces, a termination by Hf leads to a metallic and non-magnetic surface, while surfaces covered by a full monolayer of O are predicted to be half-metallic and ferromagnetic, the magnetisms being induced by the Coulomb repulsion between p-holes in the O-2p valence band. In contrast, the partially reduced surfaces terminated by half a monolayer of oxygen are found to be insulating and non-magnetic. Ab initio statistical mechanics in combination with the DFT total-energy calculations show that the partially reduced surfaces are stable over the entire range of admissible values of the chemical potential of oxygen. Investigations of the formation of Hf vacancies on the Hf- and O-terminated surfaces of tetragonal HfO_(2) demonstrate that under oxidizing conditions, the formation of Hf subsurface vacancies is energetically favored on the partially reduced O-terminated surface. The formation of Hf vacancies causes the creation of holes in the O-2p valence band and of magnetic moments on the surrounding O atoms. That the formation of near-surface Hf vacancies on the O-terminated surface is energetically favored is in contrast to a high formation energy for neutral Hf vacancies in bulk HfO2 and suggests a cooperative mechanism between surface- and vacancy-formation. We discuss our findings in relation to recent reports on ferromagnetism in ultrathin HfO_(2) films and other models for the formation of p-wave ferromagnetism

Bond formation at the Ni/ZrO_(2) interface

We report on the formation of strong chemical bonds at the Ni(100)/cubic-ZrO_(2)(100) polar interfaces. Ab initio density functional theory calculations demonstrate that both Zr/Ni and O/Ni junctions are energetically stable, and predict that two different interactions determine the interface adhesion. Our results reveal that O-Ni ionic bonds are formed by Ni electron donation, while the Zr-Ni bonds show a mixed character with ionic and electron hybridization contributions

Unexpected magnetism in low dimensional systems: the role of symmetry

The symmetry underlying the geometric structure of materials determines most of their physical properties. In low dimensional systems the role of symmetry is enhanced and can give rise to new phenomena. Here, we report on unexpected magnetism in carbon nanotubes and O-rich surfaces of ionic oxides, to show how its existence is closely related to the symmetry conditions. First, based on tight-binding models, we demonstrate that chiral carbon nanotubes present spin splitting at the Fermi level in the absence of a magnetic field, whereas achiral tubes preserve spin degeneracy. These remarkably different behaviors of chiral and non-chiral nanotubes are due to the intrinsic symmetry dependence of the spin-orbit interaction. Second, the occurence of spin-polarization at ZrO_(2), Al_(2)O_(3) and MgO surfaces is proved by means of abinitio calculations within the density functional theory. Large spin moments develop at O-ended polar terminations, transforming the non-magnetic insulator into a half-metal. The magnetic moments mainly reside in the surface oxygen atoms, and their origin is related to the existence of 2p holes of well-defined spin polarization at the valence band of the ionic oxide. The direct relation between magnetization and local loss of donor charge shows that at the origin of these phenomena is the reduced surface symmetry